The present invention relates to electro-optic displays and systems for addressing such displays and, in particular, to electro-optic display overlays.
Many types of emissive displays exist and there are many methods of fabricating emissive displays. However, most emissive displays suffer from poor contrast, viewing angle restrictions, and constant power requirements. For example, liquid crystal displays (LCD""s), which are commonly used in screens for laptop computers and other consumer electronic devices, suffer from both viewing angle restrictions, meaning that these displays can only be viewed from a few physical positions constrained by the viewing axis of the display, and limited favorable light conditions. Thus, the usefulness of emissive LCD displays is typically limited. For example, use of emissive LCD displays in high light situations (such as outdoor use) is generally impractical.
Further, nearly all emissive display require a constant supply of power to operate. This power requirement makes emissive displays challenging to incorporate in devices that do not have a connection to a constant supply of power or that have a connection only to a limited power supply such as batteries.
It is desirable to provide a display overlay that improves contrast and viewing angle for an emissive display while, at the same time, reducing power consumption.
The display overlays described later improve the contrast and remedy the viewing angle restrictions present in emissive displays as they exist today. Such display overlays will increase the usefulness of current day emissive displays and enable them to be used in more numerous situations, including high light situations. It is desirable that such display overlays improve the life and power efficiency of emissive displays. The display overlays may be added to existing emissive displays or, in other embodiments, may be designed as an integral part of emissive displays. Display overlays of the invention may be flexible. The display overlay materials may, for example, be printed onto thin, flexible substrates. Such substrates may include pliable, plastics, polymeric films, metal foils, and thin glass, for example.
Reflective electro-optic display overlays, especially encapsulated electro-optic display overlays, and systems including modules for addressing such display overlays may improve the usefulness and lifetime of present day emissive displays. Preferable systems of the invention avoid the high costs associated with using direct drive and active matrix drive addressing schemes by allowing for the addressing of display media that have poor threshold behavior. The display overlay of the invention improves the viewing angle restrictions and performance of the display in bright conditions by converting the emissive display to a reflective display. An emissive display includes emissive material (i.e., a light system that generates a pattern of light).
There are a number of interesting reflective display media which provide good optical appearance and the ability to be easily constructed in large areas or on flexible substrates at low cost. Such display media may be employed in displays and display overlays in accordance with the present invention. Suitable display media include microencapsulated electro-optic displays, electrochromic displays, rotating bichromal ball displays, suspended particle displays, and composites of liquid crystals with polymers, including polymer dispersed liquid crystals, polymer stabilized liquid crystals, and liquid crystal gels.
In one aspect of the invention, a display overlay is a reflective electro-optic display in optical communication with an emissive display. The display overlay also includes a photoconductive layer that is adjacent an electro-optic layer and a means for applying an electric field across the electro-optic layer (which may hereinafter be referred to as the xe2x80x9ccontrast mediaxe2x80x9d layer). An electro-optic layer may be a reflective electrophoretic layer. In one embodiment, the means for applying an electric field includes an electrostatic charge deposited on at least one of the electro-optic layer or the photoconductive layer. In a second embodiment, the means for applying an electric field includes a first electrode and a second electrode. The display overlay includes a synchronization module that receives signals indicative of an emissive display output and controls the application of an electric field to the display overlay in response to the signals received by the synchronization module. In one embodiment, the synchronization module controls application of light to the display overlay.
In some embodiments, the first electrode is adjacent the first side of the photoconductive layer, and the second electrode is adjacent the contrast media layer. One or more electrode of the display overlay may be light-transmissive, transparent, or translucent. The electrodes may be formed from indium tin oxide (ITO). In a particular embodiment, the electrode on the rear side of the display overlay is translucent. In another embodiment, the electrode on the rear side of the display overlay is light-transmissive. In yet another embodiment, the electrode on the rear side of the display overlay is transparent. Light from the light source travels through the rear electrode to strike the photoconductive layer. When the display overlay is exposed to an emission, such as light generated from a light source, the pattern of light reduces impedance in the photoconductive layer. The reduced impedance permits an applied electric field to address the contrast media layer. In one embodiment, the electro-optic display visual appearance is substantially similar to the visual appearance of the emissive display. In another particular embodiment, the electrode on the front side of the display overlay is translucent; a translucent front electrode may enhance viewability of the electro-optic display visual appearance. In one embodiment, the electrode on the front side of the display overlay is transparent; a transparent front electrode may also enhance viewability of the electro-optic display visual appearance. In another embodiment, the electrode on the front side of the display overlay is light-transmissive; a light-transmissive front electrode may similarly enhance viewability of the electro-optic display visual appearance. The display may include, for example, a first protective layer adjacent the first electrode and a second protective layer adjacent the second electrode.
The display overlay may be placed adjacent, or overlay, an emissive display. The emissive display may include, for example, a cathode ray tube, LCD or other emissive display such as an electroluminescent display. Alternatively, the display overlay may be integral with the emissive display.
In some embodiments the display overlay synchronization module receives signals indicative of emissive display output. Responsive to the received signals, the synchronization module controls the first and second electrodes to apply an electric field to the contrast media layer. In a first embodiment, the synchronization module controls the first and second electrode via a universal serial bus (USB). In a second embodiment, the synchronization module controls the first and second electrodes via a serial cable. In another embodiment, the synchronization module controls the first and second electrodes via an optical interface. The synchronization module may include, for example, a device driver, a thread, or WINDOWS sub-system. In other embodiments the synchronization module may be special-purpose hardware such as appropriately configured programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), or application-specific integrated circuits (ASICs).
In other embodiments, the display overlay has one or more optical barrier layers. The optical barrier layers may be placed between a second side of the photoconductive layer and the contrast media layer. Such an optical barrier layer may prevent unwanted addressing of the display overlay caused by light from, for example, the side of the display overlay from which the image is to be viewed from reducing the impedance of the photoconductive layer of the display overlay. Although the electro-optic layer used in the present invention is reflective, several types of reflective electro-optic materials do allow a small proportion of the light incident upon them to leak through, and even a small amount of such leakage could adversely affect the operation of the present overlay. For example, consider a situation in which an overlay of the present invention is being used to enable a back-lit LCD to be read easily in bright sunlight. Under such conditions, the sunlight incident upon the viewing surface of the overlay could easily be 50 times as bright as the LCD, so that leakage of, for example, 2 percent of this incident sunlight through the electro-optic layer would expose the photoconductor to light as bright as that of the LCD, essentially destroying the ability of the overlay to reproduce the image on the LCD. Alternatively or in addition, the optical barrier layer may be employed to selectively mask the display to prevent the impedance of the photoconductive layer from being reduced, thus preventing the areas where the xe2x80x9cmaskingxe2x80x9d optical barrier layer is present from being addressed. The optical barrier layer may include a conductive material.
The optical barrier layer may have anisotropic conductivity, with the conductivity through the thickness of the barrier layer being substantially greater than its conductivity laterally within the barrier layer. It will be appreciated that current passing through the photoconductor layer will tend to spread out laterally as it passes through the barrier layer, and this spreading of current causes xe2x80x9cbloomingxe2x80x9d of light-emitting areas of the emissive display, i.e., the reproduction on the contrast medium layer of a light-emitting area of the emissive display will be slightly larger than that of the original area, the difference in size being governed by the lateral spreading of current within the photoconductor layer. Such blooming is undesirable since it produces distortion of the image seen on the overlay. By using a barrier layer with anisotropic conductivity, the current spreading, and hence the blooming problem, can be minimized. Suitable barrier layers with anisotropic conductivity may comprises small metal filaments or threads oriented in one direction within a polymeric matrix; such materials are available commercially, often being referred to as xe2x80x9cz-axis conductorsxe2x80x9d.
In still other embodiments, the contrast media layer of the display overlay has at least one capsule containing a suspending fluid and a particle. In other embodiments the electro-optic layer comprises a polymeric substrate defining a plurality of cells that contain a suspending fluid and a particle. In these embodiments, the suspending fluid may include a first optical property and the particle may include a second optical property. When an electric field is applied to the electro-optic layer, the capsule changes visual states responsive to the optical properties of the suspending fluid and the particle. The particle may be, for example, substantially white, black, or red. The suspending fluid may be, for example, substantially transparent or substantially black.
In another embodiment, at least one capsule or cell in the electro-optic layer contains a first particle and a second particle. The first particle may have a first optical property and the second particle may have a second optical property. An electric field applied to the electro-optic layer causes the capsule to change visual state in response to the first and second optical properties of the first and second particles. In one embodiment where the capsule or cell contains a first particle and a second particle, at least one particle is substantially white or at least one particle is black. The capsule or cell may contain, for example, at least one red particle, at least one blue particle, and at least one green particle.
In some embodiments the photoconductive layer comprises material selected from, for example, organic photoconductive polymers, dye aggregate photoreceptors, and pigment-based photoconductors. In one particular embodiment, the photoconductive layer includes 2,4,7-trinitro-9-fluorenone complexed with poly(N-vinylcarbazole).
In another aspect, the invention provides a method of controlling a reflective display to have a visual appearance substantially similar to an emissive display. The reflective display includes an electro-optic layer adjacent a photoconductive layer. The photoconductive layer provides impedance. The reflective display includes a first electrode adjacent a first side of the photoconductive layer and a second electrode that is adjacent the electro-optic layer. According to the method, signals are received from the emissive display and the first and second electrodes are controlled to apply an electric field to the contrast media layer in response to the received signals. In one embodiment, the synchronization module controls the first and second electrodes to apply an electric field to the contrast media layer for the duration of time required for the contrast media layer to respond. A control signal may be transmitted to the first and second electrode via a USB.
In one embodiment, the emissive display output is a pattern of light displayed by the emissive display. The light pattern decreases the impedance of the photoconductive layer and an electric field is applied, addressing the contrast media layer. In some embodiments, the pattern of light and resulting display on the reflective display overlay is responsive to the emissive display output.
In yet another aspect, the invention is an article of manufacture for controlling a reflective display to have a visual appearance that is substantially similar to an emissive display. The reflective display includes a contrast media layer adjacent to a photoconductive layer. A first electrode is adjacent a first side of the photoconductive layer and a second layer is adjacent the contrast media layer. The article of manufacture comprises a computer-readable program means for receiving signals indicative of output from the emissive display. The article of manufacture also comprises a computer-readable program means for controlling the first and second electrodes to apply an electric field to the contrast media layer in response to the received signals. In one embodiment, the computer-readable program means transmits a control signal to the first and second electrodes via a USB. In another embodiment, the control signal is a xe2x80x9ctiming signalxe2x80x9d that controls the first electrode and the second electrode to provide an electric field to the contrast media layer for the time required for the contrast media layer to respond. In one embodiment, the control signal controls the first and the second electrode to provide an electric field for the time required to reset, i.e., xe2x80x9cblackenxe2x80x9d or xe2x80x9cblankxe2x80x9d, the display overlay. In another embodiment, the control signal controls the first and the second electrode to provide an electric field for the time required to address the image of the emissive display on the reflective display. In one embodiment, when the control signal remains idle the reset image may be visible. In another embodiment, when the control signal remains idle the addressed image may be visible.
The advantages of the invention described above, as well as further advantages of the invention, will be understood further upon consideration of the following drawings, description, and claims.